Bug 1729545, Try to ensure session history loads can't get stuck because of bogus requested index, r=peterv

Requested index handling is very fragile. We should try to figure out a way to remove it altogether.
(But unfortunately some behavior similar to it is needed in certain cases).
The patch is a stop-gap solution.

Differential Revision: https://phabricator.services.mozilla.com/D137479

docshell/shistory/nsSHistory.cpp

@@ -2047,7 +2047,11 @@ nsresult nsSHistory::LoadEntry(int32_t aIndex, long aLoadType,
 
   if (aIndex < 0 || aIndex >= Length()) {
     MOZ_LOG(gSHistoryLog, LogLevel::Debug, ("Index out of range"));
-    // The index is out of range
+    // The index is out of range.
+    // Clear the requested index in case it had bogus value. This way the next
+    // load succeeds if the offset is reasonable.
+    mRequestedIndex = -1;
+
     return NS_ERROR_FAILURE;
   }